1) Field of the Invention
The present invention relates to a sensing element for a catalytic combustion type gas sensor.
2) Description of the Related Art
Conventionally, a catalytic combustion type gas sensor is used as a gas sensor to detect flammable gas such as hydrogen and methane. The catalytic combustion type gas sensor includes a Wheatstone bridge circuit. The Wheatstone bridge circuit includes a sensing element, a compensating element, and two resistances.
The sensing element includes a sintered body which is kept at suitable temperature depending on a type of the target gas. On contact with the target gas, the sintered body causes combustion. A part of heat generated by the combustion is transferred to a platinum heater coil, a main part of which is embedded in the sintered body, while the rest of the heat is dissipated into atmosphere. The heat transferred to the heater coil changes temperature of the heater coil, thereby changing resistance of the heater coil. The change in the resistance is output as a voltage change of the sensing element. Thus, the inflammable gas is detected.
A sensing element including a heat conducting layer as the sintered body has been disclosed in, for example, Japanese Patent Application Laid-Open No. H3-162658. The heat conducting layer is made of aluminum nitride combined with alumina, and includes an oxidation catalyst for causing the combustion.
On the other hand, the compensating element is provided to stabilize the output voltage of the gas sensor, by canceling a voltage change of the sensing element due to a temperature change of ambient atmosphere. The compensating element has a similar structure to that of the sensing element. However, the sintered body of the compensating element includes some kind of oxide, instead of the oxidation catalyst, which does not have a combustion activity to the target gas. Therefore, the compensating element does not cause the combustion of the target gas.
With the above structure, the catalytic combustion type gas sensor can alarm a gas leak in the form of output voltage change corresponding to a temperature difference between a temperature due to the combustion and a temperature of ambient atmosphere. However, the above sensing element has the following problem due to low stability of aluminum nitride. That is, when the target gas (such as hydrogen and methane) burns, chemical reaction as shown in equation 1 occurs to generate water vapor. The water vapor reacts with aluminum nitride, thereby decomposing aluminum nitride into alumina and ammonia as shown in equation 2.2H2+O2→2H2O  (1)2AlN+3H2O→Al2O3+2NH3  (2)
Catalysis of the oxidation catalyst is temporarily reduced if ammonia exists near the sensing element. As a result, gas sensitivity of the sensing element is temporarily degraded. In addition, such combustion causes a change in a composition of the heat conducting layer gradually from an aluminum nitride base to an alumina base. According to the change, heat conductivity of the heat conducting layer changes, resulting in fluctuation of a zero point of the gas sensor. Thus, it is difficult to commercialize a catalytic combustion type gas sensor including such a sensing element disclosed in Japanese Patent Application Laid-Open No. H3-162658.
The larger a variation of output voltage of the Wheatstone bridge circuit at the same gas concentration is, the higher the gas sensitivity of the gas sensor is. To enhance the gas sensitivity, rate of the heat that is transferred to the heater coil should be increased, while decreasing rate of the heat that is dissipated into atmosphere.
Furthermore, the shorter the time required for the output voltage of the Wheatstone bridge circuit to become stable is, the higher the responsivity of the gas sensor is. To enhance the responsivity, the heat should be transferred to the heater coil through the sintered body as fast as possible.
Moreover, the zero point of the gas sensor should not fluctuate throughout the use of the gas sensor. The sintered body can be further sintered by heat generated by the heater coil to keep the sintered body at a suitable temperature, which changes physical property (such as the heat conductivity) of the sintered body and therefore the zero point of the gas sensor. To avoid the fluctuation of the zero point, the sintered body should be sufficiently sintered at the manufacturing stage, so as not to be further sintered during use of the sensing element.